The present invention relates to an anti creep device for an automatic transmission for a vehicle, and more particularly relates to such an anti creep device which can prevent anti creep action when the transmission is shifted to a higher speed stage than the first speed stage, and also to such an anti creep device which can be manually controlled to prevent or to provide creep, according to the will of the vehicle operator.
A vehicle equipped with an automatic transmission has a tendency, when stationary with the engine running in the idling condition and with its manual range selection means such as a range shifting lever set to a forward drive position such as drive range, to move forward against the intention of the vehicle operator, due to the dragging torque of the fluid coupling such as a torque converter; this is called creep. This creep phenomenon is in general undesirable, since it is outside the control of the vehicle operator and typically requires him or her to keep the brake pedal of the vehicle depressed in order to prevent vehicle motion. Also, when the vehicle is not moving, such creep applies a braking force on the engine of the vehicle, and thus necessitates so called idle up to be performed, in which the throttle valve of the engine is somewhat opened as compared to its setting when the transmission is in neutral range, in order to compensate for this braking action. This deteriorates the fuel economy of the vehicle, and also complicates the control structures therefor.
Accordingly, it has been recognized that, when the vehicle is stationary with the engine running in the idling condition and the transmission set to the drive range, it is desirable to cut off the power transfer between the engine and the drive wheels of the vehicle completely by setting the transmission to a neutral state, so as to be able to permanently keep the throttle valve set to a low setting, and so as to prevent any undesired forward motion of the vehicle; and in the past various anti creep devices have been proposed to this end. A typical such prior art device is one in which the stationary state of the vehicle is detected and the actuating pressure of a friction engagement device (such as a clutch) which provides the first speed stage is kept below its engagement pressure, so as not to engage the first speed stage at all in these conditions. Then, when the vehicle is to be moved away from rest, the operator presses down on the accelerator pedal thereof, and it is arranged that the actuating pressure of the friction engagement device is controlled in proportion to the mount of depression of said accelerator pedal. According to such a system, the vehicle speed is used as an auxiliary parameter for detecting when the vehicle is at rest. Now, this does not create any problem when the anti creep device is electrically controlled, but when the system is based upon a hydraulic system, in order to improve the comfort feeling of starting the vehicle off from rest, it is necessary to detect the vehicle speed either electrically or hydraulically. In the former case, the cost becomes high; and in the latter case it becomes necessary to introduce a hydraulic pressure type vehicle speed detector for this purpose only into an automatic transmission.
It is conceivable to use the governor valve as a vehicle speed detector, but, since a governor valve generally outputs a hydrualic fluid pressure which gradually increases from zero vehicle speed, a snap action mechanism with some hysteresis similar to those used in shift valves bcomes necessary, and again the problem arises that the cost becomes high.
Also, as a matter of course, normally anti creep is provided when the transmission is engaged to its first speed stage, and operates basically by disengaging the first speed stage friction engaging means. Now, if this anti creep function is also provided when the vehicle is running with the transmission being engaged to a speed stage higher than this first speed stage, with the first speed stage friction engaging means being disengaged in this condition also, the problem may arise of free wheeling of this first speed stage friction engaging means (depending however upon the exact particular structure of the transmission), and this can cause rubbing between the respective friction engaging members, thus generating heat and possibly damaging the friction engaging means.
Another consideration that has come to be realized is that in some circumstances creep is actually desirable. For example, it may be convenient when starting the vehicle off from rest on a slope, or when parking in a restricted space. Therefore, in these circumstances, it is desirable for the vehicle operator to be able to prevent the operation of the anti creeping device by a simple means. Now, such a means might be conceived of to be an electrical one, in which an electrical switch was available to the vehicle operator for selectively either providing or not providing anti creep action; but such a mixed electrical/hydraulic system could be unreliable, and involves cost. Accordingly it is desirable for such a system to be purely hydraulic.